The present invention relates to an improved process for the dewatering of biological sludge.
Biological sludges result from biological wastewater treatment processes such as the well-known Waste Activated Sludge Process or its modifications. Biological sludge is largely a mixture of bacteria and metabolic by-products which result from bacterial attack on the wastes in the wastewater; biological sludge also includes nondegradable or partially degraded materials.
Biological sludges are a significant disposal problem because they are extremely dilute, and hence, are generated in immense volumes on a daily basis by treatment plants. Because the volume of biological sludge generated is so high, the cost of transporting and disposal of the biological sludge is high. A primary goal of any biological sludge dewatering process is to minimize the volume. These sludges cannot be filtered directly, and therefore, filtration or other mechanical or gravitational means of sludge dewatering, such as disclosed by U.S. Pat. No. 2,850,449 are impractical without further treatment.
Thermal conditioning of biological sludge for purposes of dewatering was first reported in the early 1900's. The Porteous process, a basic thermal conditioning process, is disclosed in U.S. Pat. No. 2,075,224 and involves heat curing biological sludge at elevated temperatures and pressures in the absence of air. Thermal conditioning or heat treatment of biological sludges has been accepted, and therefore, widely practiced as a means for achieving good dewatering upon filtration; it has been reported that over 100 sludge heat treatment installation have been made in the United States alone. However, today only about five installations are still in operation because of high maintenance and operating costs partially resulting from the typical conditions required to achieve good filtration.
Typical operating procedures for thermal conditioning of biological sludge involve heating the biological sludge at temperatures of 150.degree. to 240.degree. C. in a reaction vessel under pressures of 250 to 400 psig or 15 to 90 minutes as taught by Everett, "Dewatering of Wastewater Sludge by Heat Treatment," Journal WPCF 44, 92(1972); Everett, "Recent Developments in Heat Treatment, " Wat. Pollut, Control 72, 428(1973); and Process Design Manual for Sludge Treatment and Disposal, EPA 1979. The sludge is then dewatered by conventional mechanical dewatering devices such as vacuum filters, centrifuges, belt filter presses, or plate and frame presses. The dewatering process frequently requires the addition of cationic polymer flocculants for high efficiency. Generally, the heat treatment of biological sludge and subsequent dewatering thereof provide cake solids of about 30-50%. It is well known in the prior art that chemical conditioning alone provides cake solids of only about 20%.
The problems with heat treatment of biological sludges are well-known in the art as reported by Haug et al., "Effect of Thermal Pretreatment on Digestibility and Dewaterability of Organic Sludges," Journal WPCF 50, 73(1978). One problem is the high maintenance cost as a result of corrosion caused by salt present in the sludge and operation at high temperatures and pressures. A related problem is the high capital equipment cost because expensive corrosion-resistant metals and rugged construction sufficient to meet the high temperature and pressure conditions are required. Another problem is the intense unpleasant odor associated with the hot biological sludge when it is released from the reaction vessel. The energy requirements are also high which is disadvantageous. Another problem is that the filtrate contains high Biological Oxygen Demand (B.O.D.) levels, usually on the order of 20,000 mg/l or higher, which result from the solubilization of the solids. As the heat treatment temperature increases, the amount of solids in the biological sludge decreases and the amount of dissolved B.O.D. in the filtrate increases; as such, the filtrate must be treated before the filtrate is discharged to the environment. This treatment cost is very high and is proportional to the B.O.D. level.
A need exists in the art for an improved biological sludge dewatering process wherein the disadvantages of current heat treatment processes are avoided and solids are provided in a higher percentage than when chemicals are used alone without heat treatment.